The intensity of a beam of light used to etch concentric servo tracks (12) on a magnetic medium is adjusted with an acousto-optical device to maintain the energy density delivered by the beam substantially constant while the optics (71) which generate the beam move radially of the disk (61).

A beam of light used to etch servo tracks on a magnetic disk is steered with an acousto-optical device to maintain the beam in concentrical patterns while mechanically moving the optics which generate the beam of light continuously radially of the disk.